The following article will guide you about how the fats are synthesized in plants.
(1) Synthesis of Glycerol:
There may be different methods of the formation of glycerol in plants, but one of the very common methods is from dihydroxy acetone phosphate which is an intermediate of glycolysis. Dihydroxyacetone phosphate is first reduced to α-glycerophosphate by the enzyme glycerol-3-phosphate dehydrogenase. Co-enzyme NADH2 is oxidised in this reaction.
α-glycerophophate is then hydrolysed by glycerophosphatase to liberate phosphoric acid and forming glycerol.
(2) Synthesis of Fatty Acids:
Long chain saturated fatty acids are synthesized in plants from active two carbon units, the acetyl-CoA (CH3CO.CoA). Although the reactions of β-oxidation of fatty acids are reversible, the fatty acids are not formed simply by the reverse reactions of β-oxidation. Synthesis of fatty acids from CH3CO.CoA takes place step by step. In each step the fatty acid chain is increased by two carbon atoms.
Each step involves two reactions:
(i) In the first reaction which takes place in the presence of acetyl-CoA carboxylase, acetyl-CoA combines with CO2 to form malonyl-CoA (malonic acid is 3-C compound). ATP provides energy while Mn++ and biotin are required as co-factors.
(ii) Malonyl CoA reacts with another molecule of CH3CO.CoA in the presence of fatty acid synthetase and Coenzyme NADPH to form Coenzyme-A derivative of butric acid (butric acid contains 4-C-atoms). One mol. of CO2, H2O and CoA are released while NADPH is oxidised in the reaction.
Butyryl CoA, in the next step will combine with malonyl CoA to form CoA derivative of fatty acid containing 6-C atoms. This process is repeated till Coenzyme-A derivative of long chain fatty acid (which may contain up to 16-18C atoms) is produced.
(As a matter of fact the enzyme fatty acid synthetase is not simple but a complex of many enzymes (multienzyme complex) and an acyl carrier protein called as ACP. And actually the reaction (ii) described above only summarizes a number of reactions involved in the synthesis of fatty acid from acetyl-CoA and malonyl-CoA which can be grouped under 3 categories:
(a) Initiation Reaction:
In this reaction acetyl CoA transfers its acetyl group to one of the —SH groups of multienzyme complex i.e., fatty acid synthetase.
(b) Chain Elongation Reactions:
Six different reactions involved here are:
(i) Malonyl transfer,
(vi) Acyl transfer.
Chain elongation starts with the transfer of malonyl group from malonyl-CoA to second – SH group of the multi-enzyme complex. Then, there is condensation of the latter so that a 4- C unit is produced. This unit by next three reactions i.e., reduction, dehydration and reduction is converted into a saturated 4-C unit (i.e., butyryl-CoA).
In acyl transfer reaction the fatty acid residue is transferred back to the – SH group to which the acetyl group was transferred in initiation reaction. The cycle is repeated again and again with malonyl transfer, condensation etc. till the fatty acid residue consists of up to 16-18 C atoms. Each such turn elongates fatty acid chain by 2-C atoms.
Details of chain elongation reactions are given below:
(c) Termination Reaction:
When the fatty acid residue has attained a desired length the chain elongation stops at reaction (v) and the cycle is not repeated. The acyl group instead of being transferred to the—SH of the enzyme is transferred to —SH group of Co-enzyme A (CoASH) molecule. Thus, CoA derivate of the fatty acid is produced which can then be utilised in fat synthesis. The enzyme become free.
It is believed that during this process of fatty acid synthesis, the acyl group of fatty acid is bound to the—SH group of ACP. The latter then passes it from one enzyme of the complex to the other.
(3) Condensation of Fatty Acids and Glycerol:
The fats or triglycerides are synthesized not from glycerol and free fatty acids but from a- glycerophosphate and CoA derivatives of fatty acid, i.e., fatty acyl CoA residues. First, there is acylation of α-glycerophosphate by two fatty acyl-CoA molecules to from phosphatidic acid.
Now dephosphorylation occurs in the presence of phosphatase and a diglyceride is formed.
The acylation of the free—OH group of diglyceride completes the biosynthesis of triglyceride or fat.